In liquid flow installations, particularly urban hydrology, there is a need for volumetric flow measurement systems as operational intelligence tools. Many costly, delicate, or complex methods exist for scientific investigation of steady and unsteady flow in open or closed channels. However, for various reasons prior art devices have not been acceptable for use in operational applications due to several factors including high cost and complexity; vulnerability to damage by large objects in the flow; difficulty of installation and maintenance in existing or new installations; inability to give valid data for partially filled as well as fully filled and pressurized liquid flow conditions; and risk of loss due to vandalism.